The Mistlands is... divisive. Some people love it, other people have rage quit Valheim over it. It's certainly different from all of the biomes that came before, and feels a lot more unique and fantasy than the others; all of the earlier biomes, despite having fantasy elements (like trolls, draugr, drakes and goblins) are environments you can find in our own world, whereas the Mists themselves are bizarre and a little frightening, and the lands they conceal are strange and otherworldly, full of creatures not found in our world at all; the closest thing to normal are the misthares if you can ignore the scaly hides.
It also asks that you've mastered all of the lessons from the earlier biomes: stay rested, eat good foods, manage your stamina and always be aware of your surroundings. Sounds in particular are important cues and can mean the difference between success and death.